1. Field of the Invention
The present invention relates to a method of coding images converted into digital video signals in accordance with a format composed of three-dimensional blocks of said signals.
The invention also relates to a coding arrangement for carrying out such a method and to a corresponding decoding arrangement.
This invention is essentially applied in the field of television for the transmission and/or recording of television signals. The coding arrangement according to the invention is arranged in the transmitting section of the transmission system and the corresponding decoding arrangement is arranged in its receiving section.
The transmission or recording of video signals in a digital form provides the possibility of considerably reducing the influence of channel or read noise on the quality of the displayed image and of easily including these digital signals in telephone-type digital networks. Nevertheless, the digitization of a sequence of television pictures is expressed by a very high data rate which generally cannot be directly transmitted or recorded on the existing carriers (this rate is 216 Mbit/s for digitized color television signals in accordance with notice 601 of the CCIR). It is thus essential to reduce this rate for adapting the signal to practical rates.
2. Description of Related Art
U.S. Pat. No. 4,394,774 describes a method with which this rate can be reduced by a factor of 10 to 20. This technique, which is based on the utilization of orthogonal transforms, provides the possibility of benefiting from the redundancy between neighboring samples within the image: it consists of dividing the image into blocks having an identical size and subjecting each block to an orthogonal transform which has the property of decorrelating the samples of the block by concentrating the energy on a small number of samples.
In order to benefit equally from the image-to-image redundancy present in the stationary parts of the image, this technique is often combined with an inter-frame prediction technique. According to this technique either the block itself is transmitted (intra-frame mode) or the difference between this block and the block having the same spatial position in the preceding image after coding and decoding (inter-frame mode) so that the block having the minimum energy is transmitted.
This image-to-image prediction operation thus introduces a time-recursivity of the coding operation, that is to say that it is only possible to decode an image if the preceding decoded image is available. This characteristic has, as a consequence, a risk that an error appearing at the reception or during reading of the band will subsist in various images, in fact as long as the block in which this error has appeared is coded in accordance with the inter-frame mode.
Moreover, this recursivity is incompatible with consumer video recording because it precludes random access to images, which access is necessary for realizing the "quick search" mode. In certain cases, a remedy to this drawback is to encode one image out of N in the intra-frame mode but, as this degrades the quality of displayed images, N is chosen to be large to limit this degradation, which limits the range of improvement.